A method of generating a three-dimensional image (three-dimensional volume data) from a tomographic image group, which is scanned by an X-ray Computed Tomography (CT) device and a Magnetic Resonance Imaging (MRI) device, and generating and displaying an image (hereinafter, referred to as an analysis image) which is suitable for diagnosis is known. For example, a three-dimensional volume rendering image, a Maximum Intensity Projection (MIP) image, or the like is generated as the analysis image.
However, in a case where an abdominal region is scanned using the X-ray CT device or the MRI device in such a way that a luminal organ, such as a large intestine, is expanded by carbon dioxide gas or air, cleaning in the luminal organ is generally necessary. However, it is difficult to completely clean the luminal organ and a residue often remains in the organ. In such a case, it is difficult to discriminate the residue from a lesion and it is inconvenient for image diagnosis. Here, a method called “Fecal Tagging” of imaging the residue using an oral contrast agent and distinguishing between the residue and the lesion by causing a pixel value of the residue on the image to be higher than a CT value of the lesion is used. Furthermore, in recent years, a technology (called residue removing or electronic cleansing) is used in which the contrast residue is removed from the analysis image through an image process.
In the simple residue removing, with regard to a contrast residue region, which is identified by a process of processing the pixel value as a threshold or the like, the analysis image is generated by converting the pixel value into a pixel value corresponding to air. However, at a boundary part between a plurality of substances, such as a boundary between air and a contrast residue, the pixel value continuously changes due to partial volume effect. Therefore, in a simple threshold process, it is difficult to identify and remove a boundary region between two regions (two-layered body region), and thus boundary region is drawn in the analysis image. The partial volume effect is a phenomenon in which, in a case where two regions having different pixel values are in contact with each other in a CT image, a pixel value in the boundary region continuously changes such that the pixel value connects the pixel values of the respective regions.
Here, PTL 1 proposes a method of extracting two layers and a whole boundary thereof in a two-layered body region using a fact that a contrast residue is in contact with an air region in the large intestine.